1. Field of the Invention
This invention relates to a system for online-detecting a transformation value and/or the flatness of a steel or a magnetic material.
2. Description of the Prior Art
There are various methods for strengthening steel including work hardening, precipitation hardening, solution hardening, refining of grain size, transformation structure strengthening and the like.
Backed by the recent tendency to reduce manufacturing costs of steel products, there has been activated the utilization of the transformation structure strengthening technique through the controlled cooling after the hot rolling as a measure of manufacturing a higher strength steel in a hot rolled state, using a steel blank having low alloy contents.
Now when the transformation structure strengthening technique is utilized, needless to say, it is necessary to have accurate knowledge about the transformation behavior of steel. The transformation behavior of steel, has been generally studied in an experimental manner in a laboratory. However, in many cases, the transformation behavior of steels which are manufactured by an actual system are different from the results of experiments in the laboratory to a considerable extent. Although the actual conditions have not been fully grasped yet, one of the reasons for the difference of the transformation behavior in the actual system from the results of experiments in the laboratory is said to be that the transformation behavior is complicatedly varied depending upon the history of thermal strains and the like in the preceding process or an upstream process. In consequence, if online information about the transformation behavior of steel could be precisely detected, then a great profit would be expected in the field of manufacturing various steels by means of hot rolling and heat treatment. The profit would be expected because the online information can be used, for example: (A) as a measure of grasping the variations in a local transformation behavior due to a difference in the histories of rolling temperture to improve the homogeneity in the material in the longitudinal and widthwise directions, which is a major factor governing the material quality of a hot-rolled steel; (B) as a measure of grasping a ratio between gamma phase and alpha phase at the time of applying the reduction to manufacture a steel having a high strength and being excellent in toughness, e.g. a steel plate for a high tension line pipe; or (C) as a measure of grasping a ratio of gamma phase and alpha phase at the time of starting the quenching in the manufacturing technique of a mixed structure steel to manufacture a thin steel plate having a high strength and being excellent in workability.
Heretofore, there has been proposed the following method as a method of online-detecting the above-described transformation behavior of steel. In one method, the rise in temperature due to the latent heat at the time of the phase transformation is detected by a thermometer installed on a manufacturing line to grasp a transformation behavior as described in Japanese patent publication No. 24017/81 for example. However, this method is disadvantageous in that obtainable information is given in excessively broad outline, the response is slow and the thermometer is used as a sensor, with the result that the measurement during water cooling cannot be conducted.
Furthermore, as described in Japanese patent laid-open Nos. 114518/74, 136442/76 and Japanese patent publication No. 25309/78, there have been proposed methods, in which X-rays are irradiated onto the surface of a steel, and the transformation value is measured from a diffraction strength. However, this method is disadvantageous in that it is necessary to take safety protective measure from the X-ray injuries since strong X-rays are irradiated, and consequently, the detection system itself becomes large-sized, resulting in increased installation cost, a limited installed number, occurrence of maintenance problems and the like. Additionally, since the obtainable information remains only within the surface layer portion of about 50 micro meters or less of the material to be measured, the information thus obtained cannot be used as macroscopic information.
In contrast thereto, there have been proposed methods of detecting the transformation behavior by use of a magnetic detector, utilizing the fact that the transformation from gamma phase to alpha phase of steel is accompanied by a physical phenomenon of a change from the paramagnetism (gamma phase) to the ferromagnetism (alpha phase). For example, Japanese patent laid-open Nos. 104754/75 and 82443/81 correspond to these proposals. These methods are problematical in that the measurable temperature range is limited to a temperature region of the Curie point or less of the steel to be measured. However, in most cases, the Curie points of ordinary commercial steels are as high as about 750.degree. C. In contrast thereto, the range of transformation of the steel during cooling process is transfered in non-equilibrium condition to the low temperature side, whereby the most part of transformation proceeds within a temparature range of the Curie point or less. These methods are advantageous in that the above-mentioned problems do not impose practical obstacles except for some types of steels, and are used easier in practice and the response is good as compared with other methods not utilizing the aforesaid magnetic means. Moreover, the measurements are practicable during water cooling and so on, thereby proving to practical and effective methods. However, it cannot be said that these detecting systems are satisfactory, because the magnetic detecting systems proposed by Japanese patent laid-open Nos. 104754/75 and 82448/81 have the following problems which are not solved yet:
Firstly, description will be given of Japanese patent laid-open No. 104754/75. This proposal relates to a hot rolling method utilizing a magnetic detecting system, not relating to the magnetic detecting system itself, thus the details about the fuctions and construction of the detecting system in the practical use are unclear. However, according to the description in the specification of the proposal, an excited pickup coil is placed close to the steel to be measured, and the presence of transformation is qualitatively detected from a change in impedance of the pickup coil itself generated due to the transformation of the steel to be measured from gamma phase to alpha phase. However, in order to apply the proposal to the manufacturing field of steel as aforesaid, it is necessary to have a function capable of quantitatively detecting the progress of transformation as the function of the magnetic detecting device. With the detecting device capable of only qualitatively detecting the presence of the transformation as described in the proposal, it should be said that the value of application is low.
Description will now be given of Japanese patent laid-open No. 82443/81. FIG. 1 is a block diagram showing the arrangement of the magnetic detecting system in this proposal. This magnetic detecting system is of such an arrangement that an exciting coil 3 wound around a magnetic pole 2 composed by a U-shaped iron core and magnetic flux detector 6 are spaced a predetermined distance Ls from each other. A steel 1 as being a material to be measured is interposed therebetween, and the exciting coil 3 is excited by an exciting device 4, whereby, out of magnetic fluxes 5 generated by the excitation, the intensity of a magnetic flux 5o which penetrates through the steel 1 and is leaked to reach the flux detector 6 is detected. The transformation value is detected from a change in intensity of the flux 5o generated in accordance with a transformation value in the steel 1. In the drawing, designated at 7 is a thermometer, 8 an arithmetic unit, and 9 condition signals of the material, of plate thickness and the like. Although this method has a function capable of quantitatively measuring the transformation value in principles as compared with the aforesaid Japanese patent laid-open No. 104754/75, it has the following disadvantages in the application to an actual manufacturing line.
Namely, in the above arrangement, the intensity of the magnetic flux 5o (magnetic flux density) is substantially inversely proportional to the distance Ls between the magnetic pole 2 and the magnetic flux detector 6, and hence, in order to detect the transformation value with satisfactory accuracy, it is necessary to make the distance Ls to be at least about 150 mm or less. However, it should be said that it is extremely difficult to move the steel 1 through such a narrow gap without contacting the magnetic flux detector 6 in the application of the proposal to the actual manufacturing line. This is because a hot-rolled steel plate manufactured in a hot strip mill and plate mill for example should not necessarily be flat due to buckling, camber, waving, and/or the like. This tendency is particularly strong in the forward end, rear end and therearound of the rolled steel and hence, contact of the detector 6 with the steel 1 is inevitable during conveyance of the steel. In order to avoid such an accident, there is proposed such a method that the aforesaid gap is temporarily expanded during the passing therethrough by the forward end or rear end portion of the steel for example. However, the variability in material quality tends to occur in the forward end, rear end portion or therearound, and hence, if it is impossible to detect these portions, then the value of installing the detecting device should be lost by half. Furthermore, the magnetic detecting system of the proposal is disadvantageous in that, since a vacant space portion in a magnetic circuit through which the magnetic fluxes 5 are passed is varied depending upon the difference in plate thickness t of the steel 1 as being the material to be measured, it is necessary to correct a detection output by the plate thickness t. Further, the measuring accuracy depends upon the actual plate thickness t, and so on. However, the plate thickness t of the steel 1, which is rolled by an ordinary hot rolling mill is widely varied in such ways as 1.2-30 mm in a hot strip mill and 5.0-300 mm in a plate mill for example. The correcting operations are complicated, and it is difficult from the viewpoint of the characteristics of the conveyance of the steel 1 to substantially obtain the proper detecting accuracy in accordance with the actual plate thickness t.
As described above, it should be said that the magnetic detecting system proposed by Japanese patent laid-open No. 82443/81 has many problems in the application to the actual manufacturing line.